The control of electrical actuators involving an RL type load (i.e. resistance and induction in series) may be performed either by applying a regulated voltage to the terminals of the control coil of the actuator, or by using a current source, which solution is often preferred in a severe environment since it makes it possible essentially to limit the power used for controlling the accessory and to simplify the associated corrector.
Switch-mode current sources can conventionally be considered either as being current sources that use the inductance of the load as an energy storage element during the switching operation, with the consequence of applying an alternating current (AC) voltage to the terminals of the load, which voltage alternates between positive and negative at the switching frequency, with the transitions between these two states ideally being considered as being instantaneous, or as being current sources that supply continuous current and consequently supply a continuous voltage to the terminals of the load, and where the energy storage element for switching is situated on the control circuit card itself.
Switch-mode current sources using the inductance of the load as the energy storage element have the advantage of being controllable in a manner that, at first sight, is simple. They include few or even no inductive elements, thereby leading to savings in the dimensions of the circuit. However, they present a certain number of drawbacks. They are very dependent on the inductance of the load: the ability of the switches in the current source to switch current instantaneously depends directly on the inductance of the load. It is very difficult to withstand short circuits between the outlet terminals of the converter or between either outlet terminal and ground. Specifically, in the event of the load short-circuiting, it is not possible to limit the instantaneous current, unless additional components are provided. Consequently, it is necessary in practice to add an outlet inductor to the converter in order to limit its short-circuit current, to add a protection device that switches very fast for limiting the maximum value of the short-circuit current, to add a circuit for demagnetizing the outlet inductor to manage switching off after detecting an outlet short circuit, and to overdimension the interfaces (inlet filter capacitor) so that they can withstand the short-circuit current. Concerning electromagnetic compatibility (essentially by conduction), these converters are difficult to make compatible with aviation standards, if it is desired to have a high switch-mode frequency in order to limit the overall size of the passive components of the converters, particularly if the load is controlled at the end of several meters of cable. This leads to a reduced switch-mode frequency that is typically lower than 10 kilohertz (kHz) and to the need to design an outlet filter (both common mode and differential mode) that plays a major role on overall stability and that presents non-negligible size. That type of switch-mode current source is restricted to high-power applications for which a low switch-mode frequency is not necessarily a handicap. For converters delivering a continuous voltage to the terminals of the load, switching no longer takes place in the load, and the current (or the voltage) is regulated at the outlet of a switch-mode converter having an inductor that stores at least all of the energy that is transferred to the load and a capacitor is added in order to smooth the outlet voltage. Consequently, the outlet voltage is practically continuous at the terminals of the load. There is thus less difficulty in complying with aviation standards for noise transmitted by conduction. In the event of the load short circuiting, the current through the converter naturally remains limited. It is possible to envisage switch-mode frequencies exceeding 100 kHz, limited specifically by the efficiency of the converter and by the performance of the grid control circuits of the switch elements.
FIG. 1A is a circuit diagram of a prior art converter delivering a continuous voltage to the terminals of a load. The circuit is powered by a positive voltage Vp (e.g. +25 volts (V)) and by a negative voltage Vm (e.g. −25 V), both relative to ground. It has two elements T1 and T2, each having two magnetically coupled windings around a magnetic core. The windings of a given element T1 or T2 are wound in opposition, as indicated by dots in FIG. 1A. The winding E1 of the element T1 has a first end connected to the voltage Vp via a diode D5 that is reverse-connected relative to the voltage Vp, its second end being connected to ground. The winding E2 of the element T1 possesses a first end connected to a first terminal of a switch Q3 having its second terminal connected to the voltage Vo. The second end of the winding E2 is connected to the outlet terminal S1P of the circuit. The winding E3 of the element T2 possesses a first end connected to the voltage Vm via a diode D6 that is reverse-connected relative to the voltage Vm, its second end being connected to ground. The winding E4 of the element T2 possesses a first end connected to a first terminal of a switch Q4 having its second terminal connected to the voltage Vm. The second end of the winding E4 is connected to the outlet terminal S1P of the circuit. A smoothing capacitor C1 is connected between the outlet S1P and ground.
The converter is transformed into a current source by adding means for measuring the outlet current, and an appropriate regulator and an appropriate modulator. This is shown in FIG. 1B where the load connected to the outlet of the converter is represented in the form of a resistance Rc and an inductance Lc connected in series. The outlet current is measured by measurement means 1 that deliver a representative signal to a first inlet of regulator (or corrector) means 2. A second input Ec of the regulator means 2 receives a setpoint signal. The output signal from the regulator means 2 is applied to the input of a modulator 3 that delivers a control signal SQ3 to the switch Q3 and a control signal SQ4 to the switch Q4.
The converter shown in FIGS. 1A and 1B thus has four windings and two magnetic cores, thereby leading to relatively high costs and to an overall size of the circuit that is relatively large.
In order to reduce the cost and the size of such a converter, patent application EP 1 959 549 in the name of the present Applicant proposes a circuit having two windings coupled on a single magnetic core.
Nevertheless, in that circuit, the two windings present different numbers of turns in order to avoid possible problems of cross conduction. Unfortunately, that makes it necessary to have recourse to specific elements in order to create the windings, such that the converter continues to be of relatively high cost.